tommath/bn_fast_s_mp_mul_digs.c

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#include "tommath_private.h"
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#ifdef BN_FAST_S_MP_MUL_DIGS_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
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* SPDX-License-Identifier: Unlicense
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*/
/* Fast (comba) multiplier
*
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* This is the fast column-array [comba] multiplier. It is
* designed to compute the columns of the product first
* then handle the carries afterwards. This has the effect
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* of making the nested loops that compute the columns very
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* simple and schedulable on super-scalar processors.
*
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* This has been modified to produce a variable number of
* digits of output so if say only a half-product is required
* you don't have to compute the upper half (a feature
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* required for fast Barrett reduction).
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*
* Based on Algorithm 14.12 on pp.595 of HAC.
*
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*/
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int fast_s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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{
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int olduse, res, pa, ix, iz;
mp_digit W[MP_WARRAY];
mp_word _W;
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/* grow the destination as required */
if (c->alloc < digs) {
if ((res = mp_grow(c, digs)) != MP_OKAY) {
return res;
}
}
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/* number of output digits to produce */
pa = MIN(digs, a->used + b->used);
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/* clear the carry */
_W = 0;
for (ix = 0; ix < pa; ix++) {
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int tx, ty;
int iy;
mp_digit *tmpx, *tmpy;
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/* get offsets into the two bignums */
ty = MIN(b->used-1, ix);
tx = ix - ty;
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/* setup temp aliases */
tmpx = a->dp + tx;
tmpy = b->dp + ty;
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/* this is the number of times the loop will iterrate, essentially
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while (tx++ < a->used && ty-- >= 0) { ... }
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*/
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iy = MIN(a->used-tx, ty+1);
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/* execute loop */
for (iz = 0; iz < iy; ++iz) {
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_W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
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}
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/* store term */
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W[ix] = (mp_digit)_W & MP_MASK;
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/* make next carry */
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_W = _W >> (mp_word)DIGIT_BIT;
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}
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/* setup dest */
olduse = c->used;
c->used = pa;
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{
mp_digit *tmpc;
tmpc = c->dp;
for (ix = 0; ix < pa; ix++) {
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/* now extract the previous digit [below the carry] */
*tmpc++ = W[ix];
}
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/* clear unused digits [that existed in the old copy of c] */
for (; ix < olduse; ix++) {
*tmpc++ = 0;
}
}
mp_clamp(c);
return MP_OKAY;
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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */